This invention relates to telephones. More particularly the invention relates to a solid state switch for key telephones which is adapted for either two-wire or four-wire operation.
In a typical two-wire telephone circuit both transmitted and received information is propagated on a single pair of conductors. A balance network is utilized for impedance matching and predetermined mixing of the speech generated by the transmitter with the incoming signal being delivered to the receiver. A conventional electromechanical switching relay is employed by prior art devices to couple the receiver with the balance network for mixing in the two-wire mode.
In four-wire telephone systems separate pairs of conductors are utilized for transmitting and receiving information. Four-wire operation is particularly advantageous in long distance circuits where repeaters and line amplifiers are necessary, because, for example, operating stability of such equipment is enhanced by processing information in single directions only. In four-wire systems mixing of received and transmitted information may be necessary. When required the mixing is performed at the telephone company central office, instead of utilizing the local balance network as in two-wire systems. Accordingly, the aforementioned switching relay is utilized in the four-wire mode to isolate the receiver from the balance network, and to connect it instead to the four-wire receive pair of conductors.
The prior art four-wire switching relays are undesirable in several respects. Due to their physical size, such prior art electromechanical relays are difficult to install in telephone instruments, requiring a plurality of mounting holes and considerable space. Relay connecting wires must either be spliced to the appropriate telephone wires or tied down on spare terminal board punchings. Splices are time consuming and bulky. The use of spare punchings requires a large number of design drawings to cover different telephone types. The complexity of such relays contributes significantly to their expense. Finally, since they are actuated by an electric coil, energy is unnecessarily wasted.
It is thus a fundamental object of this invention to provide a solid state replacement for the aforementioned electromechanical switching relay.
Another object is to provide a solid state relay of the character described which is compatible with either two-wire or four-wire operation.
Another object of this invention is to minimize the amount of energy consumed during four-wire operation.
A still further object of this invention is to provide a switching relay having the aforementioned characteristics which may be quickly and easily installed in a conventional telephone.
The present invention comprises a compact, solid state switch which is designed to replace the aforementioned electromechanical switching relay. The switch comprises a pair of transistors which, in the two-wire mode, electrically connect a telephone receiver with the telephone unit balance network. In the four-wire mode the transistors are biased off by a negative potential derived from the four-wire station pickup relay, so that the switch thus isolates the receiver from the telephone balance network. The circuit elements are housed within a compact, generally cubicle plastic module which is adapted to be attached to a perforated mounting surface within the telephone unit. A single mounting screw is provided on the module for the latter purpose. Installation of the module requires no splices and uses no spare telephone punchings. All connections to the telephone circuitry are made either with spade tipped module leads or via screw down terminals located on the module itself.
Other and further objects of this invention will become apparent in the course of the following description.